Normally, a permanent magnet motor refers to a motor that uses a permanent magnet as a field magnet. In the permanent magnet motor, a stator is composed of a core and a coil, and the coil is wound around a slot of the core. An insulator surrounds the outer surface of the coil so as to prevent a short circuit between adjacent coils.
However, as time progresses, the insulator may deteriorate because of a high voltage and heat. Unfortunately, this causes a short circuit (also referred to as a turn short) between adjacent coils. If such a short circuit occurs, the performance of the permanent magnet motor is degraded. Namely, the short-circuited coils are formed into a single circuit, and a high current is generated due to the magnetic flux of the stator. The high current causes copper loss, which leads to an increase in heat generation, which accelerates the breakdown of the insulator. Eventually, the permanent magnet motor may be destroyed so as not to operate, and sometimes a fire may occur. Therefore, it is necessary to detect and cope with the occurrence of a short circuit in the permanent magnet motor at an early stage.
Meanwhile, typically, a lot of time is consumed depending on a lot of tests in determining the criterion for detecting whether the permanent magnet motor is short-circuited. Also, it is difficult to detect a fault of the permanent magnet motor while the permanent magnet motor is running.